1. Field of the Invention
The present invention relates to an electric motor device, and more particularly to a starting motor for an internal combustion engine of an automotive vehicle, which comprises a cup-shaped rear bracket made of resin coupled to the central cylindrical yoke as part of its housing structure.
2. Description of the Prior Art
The starting motor for an internal combustion engine of an automobile, etc., usually comprises a D.C. electric motor accomodated in a cylindrical yoke, and an output portion (including the pinion engaging with the ring gear of the engine) accommodated in the front frame coupled to the yoke, the rear side (the side opposite to the output side) of the yoke being closed by a cup-shaped rear bracket. The rear bracket is made of a resin for the purpose of reducing the weight thereof and is secured to the yoke by means of a through bolt. Thus, the rear bracket is liable to failures due to the bending and tensile stresses developed therein, resins usually exhibiting peculiar weakness against such stresses. Further, as the rear brackets are placed in a high temperature environment for a long period of time, they are prone to be deformed continuously by the phenomenon called creep under the fastening force.
FIG. 1 shows an axial sectional view of the rear portion of a conventional starting motor for an internal combustion engine. Armature 1, which includes armature coils 1b wound around the armature core and coupled to commutator 1a, opposes field permanent magnets 2 fixed to the inner circumferential surface of yoke 3 for generating magnetic field. Cup-shaped rear bracket 4 made of a resin is coupled at the marginal portion thereof to the rear end of the cylindrical yoke 3 by a socket and spigot joining structure and is secured thereto by through bolt 5 which, extending through boss 4a formed on the outer side surface of the rear bracket 4, is fastened to the unillustrated front frame of the starting motor at the threaded end thereof. The front frame 6 is coupled to the front end of the yoke 3 for rotatably supporting the output portion of the starting motor including the pinion gear (not shown) engaging with the ring gear of an internal combustion engine. Brush 7, which is slided into and held by brush holder 7a mounted to the bottom portion of the cup-shaped bracket 4 through base plate 7b, slidably contacts the commutator 1a to supply an electric power to the armature coils 1b. The armature shaft 1c is rotatably supported by bearing 8 mounted in the central recess of the bracket 4.
The housing structure shown at FIG. 1 suffers from the following disadvantages. As the through bolt 5 extends through the boss 4a of the rear bracket 4 to fasten it to the yoke 3, portions A and B are subjected to bending and tensile stresses, respectively. The bracket 4, however, is made of resin, which generally exhibits peculiar weakness against bending and tensile stresses, although relatively strong against compressive stresses. As a result, these portions A and B of the bracket 4 subjected to the bending and tensile stresses are prone to form fractures and cracks therein; in an extreme case, the bracket 4 may be broken due to the bending of the through bolt 5. Further, the brush 7 must be provided with a separate grounding circuit, because the base plate 7b thereof is mounted to the rear bracket 4 made of an electrically insulating material, i.e. a resin. Thus, it is necessary to provide the brush 7 with a lead wire (not shown) to connect it to a metallic member such as the yoke 3. This results in an increased number of assembling steps and an increased production cost of the starting motor.
Stresses under a high temperature condition cause similar problems in other conventional starting motors having a rear bracket made of resin fastened by a through bolt. FIG. 2 shows the rear portion of another conventional starting motor for an internal combustion engine of an automobile, which has a structure similar to that shown in FIG. 1, whereby like reference numerals represent like or corresponding parts in both figures. However, the through bolt 5, extending at the head 5b thereof through the bottom portion 4d of the cup-shaped rear bracket 4, runs within the bracket 4 and the yoke 3 and is screwed to the front frame, i.e. the front bracket, which is not shown in the figure, of the starting motor. The figure further shows the lead 7d connecting the brush holder 7a to an electromagnetic switch (not shown) to supply electric power.
The housing structure shown in FIG. 2 suffers the following disadvantage. Due to the fastening force applied by the through bolt 5 on the bracket 4, the side portion 4e of the bracket 4 is subjected to a force which tends to open (i.e. expand radially outwardly) the end portion of the bracket 4. Further, as the starting motor is installed near the internal combustion engine of the automobile, it is liable to be heated to a high temperature (e.g. to about 130 degrees centigrade) for a long period of time. Thus, the rear bracket 4, which is made of a thermoplastic resin, is continuously deformed due to the phenomenon called creep; as a result, the open end portion of the bracket 4 is radially extended and the fastening force of the bolt 5 is diminished.
In view of the above-mentioned disadvantage of the structure of FIG. 2, a joining structure of the rear bracket shown in FIG. 3 has been proposed, wherein the radial positions of the annular recesses 3b and 4b of the socket and spigot joint formed at the ends of the yoke 3 and the bracket 4, respectively, are reversed with respect to the radial positions thereof in FIG. 2, so that the projection 4c at the end portion of the bracket 4 is contained from without by the projection 3c of the yoke 3. In the structure shown in FIG. 3, however, it is difficult to increase the thickness of the side portion 4e of the bracket 4 in the radially inward direction without bringing about interference with the through bolt 5 and the spring for the brush (not shown). Thus, the thickness of the projection 4c of the bracket 4 needs must be limited to a small dimension, so that it can hardly bear the stresses developed therein, especially under a high temperature condition promoting the progress of creeps in thermoplastic resins under which the starting motor is placed for a long period of time. Thus, failures are prone to develop thereat to diminish the reliability of the device.
The housing structures of electric motor devices including a rear bracket similar to those described above are disclosed, for example, in U.S. Pat. No. 4,538,085 or European Patent 0,098,992B1, which are deemed to suffer from aforementioned disadvantages provided that the rear brackets thereof are made of resin.